Claims spreading online assert that the percentage next to rainfall on a weather website or app actually means what percentage of a designated area that will see rainfall, as opposed to the overall chance of rainfall within that designated area.
The claims spread largely within the UK after a handful of celebrities posted on their social media channels that their respective “minds were blown” after discovering that the percentage next to rainfall didn’t actually refer to the chance of rainfall within an area, rather what percentage of that area will experience rainfall.
However, for many popular meteorological entities at least, these claims are largely inaccurate.
Firstly, the Met Office in the UK debunked the claim through Alex Deakin, a meteorologist and weather forecaster who responded directly to the claim on the Met Office’s Facebook page –
Certainly in the UK on the Met Office app, the percentage of rainfall means the likelihood of rain at that particular time at that particular location. So 60 percent of rain means there’s a 40 percent chance of it being dry.
The BBC uses an identical model when denoting the rainfall percentage.
Each breakdown tells you the percentage chance of it raining where you are at each hour mark, but what does a 40% chance of rain at 14:00 mean?
So, if you imagine putting a measuring jug out in your garden at 13:00, and you go and check it at 14:00, the 40% chance of rain at 14:00 is the chance of you having some rain in your jug at 14:00.
On the BBC site and app, it clarifies this by writing “Chance of Precipitation” next to the percentage number.
The National Weather Service in the US also uses the percentage number to denote the chance of at least some precipitation occurring at any single point within the designated area. In fact the National Weather Service specifically debunks the claim that it refers to the percentage the area within a designated area seeing rainfall.
To summarize, the probability of precipitation is simply a statistical probability of 0.01″ inch of more of precipitation at a given area in the given forecast area in the time period specified. Using a 40% probability of rain as an example, it does not mean (1) that 40% of the area will be covered by precipitation at given time in the given forecast area or (2) that you will be seeing precipitation 40% of the time in the given forecast area for the given forecast time period.
Let’s look at an example of what the probability does mean. If a forecast for a given county says that there is a 40% chance of rain this afternoon, then there is a 40% chance of rain at any point in the county from noon to 6 p.m. local time.
As such, the above weather services use the rainfall percentage to denote how likely it will rain any any single point in time (within the time range) at any given point (within the given area.) It does not refer to the percentage of the area that will experience rainfall, nor does it refer to the percentage of time within the time range that will experience rainfall.
Now, some weather apps do use the “percentage of an area” model to denote rainfall percentage, but this is rare in the UK where this rumour seems to be spreading most prolifically, and the majority of mainstream sites and apps we investigated did not use this method.
As such, broadly claiming that the rainfall percentage refers to the percentage of an area seeing rainfall is largely inaccurate, and we have marked it as mostly false.